Positive active material for rechargeable lithium battery, method of preparing same, and rechargeable lithium battery including same

ABSTRACT

A positive active material for a rechargeable lithium battery includes a nickel-based composite oxide represented by the following Chemical Formula 1, wherein the nickel-based composite oxide includes an over lithiated oxide and non-continuous portions of a lithium nickel cobalt manganese oxide on a surface of the over lithiated oxide.
 
Li a Ni b Co c Mn d O 2   Chemical Formula 1
         where 1&lt;a&lt;1.6, 0.1&lt;b&lt;0.7, 0.1&lt;c&lt;0.4, and 0.1&lt;d&lt;0.7.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2012-0097887, filed in the Korean IntellectualProperty Office on Sep. 4, 2012, the entire contents of which areincorporated herein by reference.

BACKGROUND

1. Field

This disclosure relates to positive active material for a rechargeablelithium battery, a method of preparing the same, and a rechargeablelithium battery including the same.

2. Description of the Related Art

Rechargeable lithium batteries have recently drawn attention as a powersource for small portable electronic devices. They use an organicelectrolyte solution and thereby, have a discharge voltage that is atleast two times higher than that of conventional batteries using analkali aqueous solution, and as a result, they have relatively highenergy density.

A rechargeable lithium battery is manufactured by injecting anelectrolyte into an electrode assembly. The electrode assembly includesa positive electrode including a positive active material capable ofintercalating/deintercalating lithium ions and a negative electrodeincluding a negative active material capable ofintercalating/deintercalating lithium ions.

As for the positive active material, LiCoO₂ has been widely used,however, it has a high manufacturing cost and an unstable supply due tothe scarcity of cobalt (Co). Accordingly, a positive active materialincluding Ni (nickel) and/or Mn (manganese) has been developed.

The positive active material using Ni (nickel) may be appropriately usedfor a high-capacity and high voltage battery but has an unstablestructure and thus, deteriorates capacity and in addition, reacts withan electrolyte and thus, it has low thermal stability.

On the other hand over lithiated oxides (which may include Mn oxides)release lithium ions at a voltage of greater than or equal to about 4.55V and thus, they have an increased capacity. However, the over lithiatedoxides generate oxygen gas during the reaction and also, have a reducedinitial efficiency because the lithium ions released from anirreversible reaction do not reparticipate in the reaction.

SUMMARY

Aspects of embodiments of the present invention are directed to apositive active material for a rechargeable lithium battery having highelectrical conductivity that suppresses the generation of oxygen gasduring charge and discharge, and also suppresses a reaction with anelectrolyte and thus, a rechargeable lithium battery including thepositive active material has high thermal stability.

Another embodiment of the present invention is directed to a method ofpreparing the positive active material for a rechargeable lithiumbattery.

Yet another embodiment of the present invention is directed to arechargeable lithium battery including the positive active material.

According to one embodiment of the present invention, a positive activematerial for a rechargeable lithium battery includes a nickel-basedcomposite oxide represented by the following Chemical Formula 1, whereinthe nickel-based composite oxide includes over lithiated oxide andnon-continuous (uncontinuous) portions of lithium nickel cobaltmanganese oxide on a surface of the over lithiated oxide, an atomicweight ratio of Ni:Mn of the over lithiated oxide is in a range of about1:1 to about 2:1, and an atomic weight ratio of Ni:Mn of the lithiumnickel cobalt manganese oxide is in a range of about 3:1 to about 4:1:Li_(a)Ni_(b)Co_(c)Mn_(d)O₂  Chemical Formula 1

wherein, 1<a<1.6, 0.1<b<0.7, 0.1<c<0.4, and 0.1<d<0.7.

The over lithiated oxide may include a compound represented by thefollowing Chemical Formula 2:xLi₂MnO₃·(1−x)LiNi_(e)Co_(f)Mn_(g)O₂  Chemical Formula 2

wherein, 0<x<1, 0<e<1, 0<f<1, 0<g<1, and e+f+g=1.

The over lithiated oxide may be included in an amount of about 5 wt % toabout 60 wt % based on the total weight of the nickel-based compositeoxide. Or, the over lithiated oxide may be included in an amount ofabout 15 wt % to about 50 wt % based on the total weight of thenickel-based composite oxide.

The lithium nickel cobalt manganese oxide may include a compoundrepresented by the following Chemical Formula 3:Li_(i)Ni_(j)Co_(k)Mn_(l)O₂  Chemical Formula 3

wherein, 0.95<i<1.05, 0.4≦j≦0.8, 0.1≦k≦0.3, and 0.1≦l≦0.3.

The lithium nickel cobalt manganese oxide may be included in an amountof about 40 wt % to about 95 wt % based on the total weight of thenickel-based composite oxide. Or, the lithium nickel cobalt manganeseoxide may be included in an amount of about 50 wt % to about 85 wt %based on the total weight of the nickel-based composite oxide.

The atomic weight ratio of Ni:Mn of the over lithiated oxide may be in arange of about 1:1 to about 1.7:1, and an atomic weight ratio of Ni:Mnof the lithium nickel cobalt manganese oxide may be in a range of about3:1 to about 3.5:1.

According to another embodiment of the present invention, a method ofpreparing a positive active material for a rechargeable lithium batteryincludes co-precipitating a first nickel (Ni) raw material, a firstcobalt (Co) raw material, a first manganese (Mn) raw material, ammoniumhydroxide (NH₄OH), and sodium hydroxide (NaOH) to obtain a firstprecipitate. The first precipitate is mixed with a lithium raw materialto obtain a first mixture. The first mixture is heat-treated. Then, theheat treated first mixture, a second nickel (Ni) raw material, a secondcobalt (Co) raw material, and a second manganese (Mn) raw material,ammonium hydroxide (NH₄OH), and sodium hydroxide (NaOH) areco-precipitated to form a second precipitate on a surface of the heattreated first mixture. The resulting material, i.e., the secondprecipitate on a surface of the heat treated first mixture and thelithium raw material are mixed to obtain a second mixture. The secondmixture is then heat-treated to obtain the nickel-based composite oxiderepresented by the above Chemical Formula 1.

The heat treating the first mixture and the heat treating the secondmixture may each independently be performed at about 890° C. to about1000° C.

According to yet another embodiment of the present invention, arechargeable lithium battery includes a positive electrode including thepositive active material; a negative electrode; and an electrolyte.

Hereinafter, further embodiments of this disclosure will be described indetail.

According to aspects of embodiments of the present invention, thepositive active material has high electrical conductivity and suppressesgeneration of oxygen gas and a reaction of the active material with anelectrolyte and thus, realizes high thermal stability and accordingly,may realize a rechargeable lithium battery having improved performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view depicting a structure of a positive activematerial according to one embodiment.

FIG. 2 is a schematic view depicting a structure of a lithiumrechargeable battery according to one embodiment.

FIGS. 3 and 4 are scanning electronic microscope (SEM) photographs of apositive active material according to Example 1 and Comparative Example1, respectively.

FIG. 5 is an energy dispersive X-ray (EDX) graph showing the inside ofthe positive active material according to Example 1.

FIG. 6 is an energy dispersive X-ray (EDX) graph showing the surface ofthe positive active material according to Example 1.

FIG. 7 is an X-ray diffraction analysis (XRD) graph showing the positiveactive material according to Example 1.

FIGS. 8 and 9 are charge and discharge graphs of half-cells including apositive active material according to Example 1 and Comparative Example1, respectively.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure will hereinafter bedescribed in detail. However, these embodiments are only exemplary, andthis disclosure is not limited thereto.

The positive active material according to one embodiment includesnickel-based composite oxide represented by the following ChemicalFormula 1:Li_(a)Ni_(b)Co_(c)Mn_(d)O₂  Chemical Formula 1wherein, 1<a<1.6, 0.1<b<0.7, 0.1<c<0.4, and 0.1<d<0.7.

The nickel-based composite oxide represented by the above ChemicalFormula 1 may include an over lithiated oxide and a lithium nickelcobalt manganese oxide coated on a surface of the over lithiated oxide.

The over lithiated oxide may be a compound represented by the followingChemical Formula 2:xLi₂MnO₃·(1−x)LiNi_(e)Co_(f)Mn_(g)O₂  Chemical Formula 2wherein, 0<x<1, 0<e<1, 0<f<1, 0<g<1, and e+f+g=1.

The positive active material that includes the over lithiated oxide mayrealize a rechargeable lithium battery having high-capacity at a highvoltage of about 4.55 V.

The over lithiated oxide may be included in an amount of about 5 wt % toabout 60 wt % based on the total weight of the nickel-based compositeoxide. In some embodiments, the over lithiated oxide may be included inan amount of about 15 wt % to about 50 wt % based on the total weight ofthe nickel-based composite oxide. In some embodiments, when the overlithiated oxide is included within the range, capacity per unit mass issignificantly increased.

The lithium nickel cobalt manganese oxide may include a compoundrepresented by the following Chemical Formula 3.Li_(i)Ni_(j)Co_(k)Mn_(l)O₂  Chemical Formula 3wherein, 0.95<i<1.05, 0.4≦j≦0.8, 0.1≦k≦0.3, and 0.1≦l≦0.3.

The positive active material includes lithium nickel cobalt manganeseoxide coated on the surface of the over lithiated oxide and thus, thegeneration of oxygen gas during charge and discharge and reaction of theactive material with an electrolyte is suppressed, thereby realizing arechargeable lithium battery having high thermal stability.

The lithium nickel cobalt manganese oxide may be included in an amountof about 40 wt % to about 95 wt % based on the total weight of thenickel-based composite oxide. In some embodiments, the lithium nickelcobalt manganese oxide may be included in an amount of about 50 wt % toabout 85 wt % based on the total weight of the nickel-based compositeoxide. In some embodiments, when the lithium nickel cobalt manganeseoxide is included within the above described range, generation of oxygengas during charge and discharge of the negative active material issuppressed, and thus, a rechargeable lithium battery havinghigh-capacity and high thermal stability is realized.

The structure of the nickel-based composite oxide may be understoodreferring to FIG. 1.

FIG. 1 is a schematic view depicting a structure of a positive activematerial according to one embodiment.

Referring to FIG. 1, the positive active material 10 may have astructure whereby over lithiated oxide 12 is coated with the lithiumnickel cobalt manganese oxide 14 on the surface. The coating may beperformed in an island shape as shown in FIG. 1, i.e., non-continuousportions of lithium nickel cobalt manganese oxide may be on a surface ofthe over lithiated oxide 12.

When the positive active material includes lithium nickel cobaltmanganese oxide coated on the over lithiated oxide, the positive activematerial may be suppressed from reacting with an electrolyte. Inaddition, when the positive active material includes lithium nickelcobalt manganese oxide having high electrical conductivity on thesurface, high-capacity at high current density may be obtained.

Because the nickel-based composite oxide has a structure including anover lithiated oxide coated with a lithium nickel cobalt manganese oxideon the surface, manganese and nickel components in the nickel-basedcomposite oxide may have a structural concentration gradient.Specifically, the nickel-based composite oxide includes a higherconcentration of nickel on the surface than at the interior.

More specifically, Ni:Mn inside or at an interior of the nickel-basedcomposite oxide (i.e., the Ni:Mn of the over lithiated oxide) may havean atomic weight ratio of about 1:1 to about 2:1. In some embodiments,the Ni:Mn atomic weight ratio inside the nickel-based composite oxidemay be about 1:1 to about 1.7:1. The atomic weight ratio of Ni:Mn on thesurface or at an outside of the nickel-based composite oxide (i.e., theNi:Mn atomic weight ratio of the lithium nickel cobalt manganese oxide)may be about 3:1 to about 4:1. In some embodiments, the Ni:Mn atomicweight ratio on the surface of the nickel-based composite oxide may beabout 3:1 to 3.5:1. In some embodiments, when nickel and manganese arerespectively included within the atomic weight ratio ranges both inside(i.e., at an interior of) and on the surface (i.e., at an outside) ofthe nickel-based composite oxide, the positive active material issuppressed from reacting with an electrolyte and in addition,high-capacity at high current density due to the lithium nickel cobaltmanganese oxide having high conductivity on the surface may be realized.In other words, in some embodiments, when the Ni:Mn atomic weight ratioof the over lithiated oxide and the Ni:Mn atomic weight ratio of thelithium nickel cobalt manganese oxide are within the above ranges, theabove benefits may be realized.

The positive active material may be prepared by the following method. Insome embodiments, the method includes co-precipitating each metal rawmaterial including nickel (Ni), cobalt (Co), and manganese (Mn),ammonium hydroxide (NH₄OH), and sodium hydroxide (NaOH) to obtain afirst precipitate. The first precipitate is then mixed with a lithiumraw material to obtain a first mixture. Then, the method includesheat-treating the first mixture, co-precipitating the heat treated firstmixture with each metal raw material including nickel (Ni), cobalt (Co),and manganese (Mn), the ammonium hydroxide (NH₄OH), and the sodiumhydroxide (NaOH) to form a second precipitate on a surface of the heattreated first mixture. Then, the resulting material, i.e., theheat-treated first mixture having the second precipitate on a surfacethereof (the second precipitate on a surface of the heat treated firstmixture) and the lithium raw material are mixed to obtain a secondmixture, and the second mixture is heat-treated to obtain thenickel-based composite oxide represented by the above Chemical Formula1.

The heat-treated first mixture may be the aforementioned over lithiatedoxide.

Herein, each of the co-precipitation reactions may be performed at a pHof about 10 to about 12, a temperature of about 35° C. to about 40° C.,and at a reaction speed of about 600 rpm to about 800 rpm for about 8hours to about 10 hours.

The lithium raw material may include lithium carbonate, lithium acetate,lithium hydroxide, and/or the like. The metal raw material may include ametal-containing acetate, a metal-containing nitrate, a metal-containinghydroxide, a metal-containing oxide, a metal-containing sulfate, and/orthe like. However, the lithium raw material and the metal raw materialare not limited thereto. In some embodiments, the metal raw material isa metal-containing sulfate. The co-precipitation may be performed in asolvent. The solvent may be water, ethanol, methanol, acetone, and/orthe like.

Each of the heat treatments may be performed at about 890° C. to about1000° C. In some embodiments, each of the heat treatments may beperformed at about 900° C. to about 950° C. The first and second heattreatments may be performed at the same or different temperatures. Insome embodiments, the heat-treatments may be performed while thetemperature is increased, for example, the heat-treatments may beperformed by increasing the temperature to 1° C./min to 5° C./min untilthe temperature reaches to about 890° C. to about 1000° C. In someembodiments, when the heat treatment is performed within the abovetemperature range, the positive active material has primary andsecondary particle sizes that provide optimum (e.g., maximum) capacityand in addition, crystallinity of the over lithiated oxide ismaintained.

Hereinafter, a rechargeable lithium battery including the positiveactive material is illustrated referring to FIG. 2.

FIG. 2 is a schematic view depicting a structure of a lithiumrechargeable battery according to one embodiment.

Referring to FIG. 2, a rechargeable lithium battery 100 according to oneembodiment includes a positive electrode 114, a negative electrode 112facing the positive electrode 114, a separator 113 interposed betweenthe negative electrode 112 and the positive electrode 114, anelectrolyte (not shown) impregnating the separator 113, a battery case120, and a sealing member 140 sealing the battery case 120.

The positive electrode 114 includes a current collector and a positiveactive material layer disposed on the current collector. The positiveactive material layer includes a positive active material, a binder, andoptionally a conductive material.

The current collector may be Al, but it is not limited thereto.

The positive active material may be the nickel-based composite oxidedescribed above. When the nickel-based composite oxide is used as apositive active material, the positive active material has highelectrical conductivity and the generation of oxygen gas and a reactionwith an electrolyte is suppressed, and thus, realizes high thermalstability and accordingly, a rechargeable lithium battery including thenickel-based composite oxide may have improved performance.

The binder improves binding properties of the positive active materialparticles to each other and to a current collector. Examples of thebinder include at least one selected from polyvinyl alcohol,carboxylmethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose,polyvinylchloride, carboxylated polyvinyl chloride, polyvinylfluoride,an ethylene oxide-containing polymer, polyvinylpyrrolidone,polyurethane, polytetrafluoroethylene, polyvinylidene fluoride,polyethylene, polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and/or the like,however, the binder is not limited thereto.

The conductive material is used in order to improve conductivity of anelectrode. Any electrically conductive material may be used as aconductive material unless it causes a chemical change. Examples of theconductive material include a carbon-based material such as naturalgraphite, artificial graphite, carbon black, acetylene black, ketjenblack, a carbon fiber, and/or the like; a metal-based material of metalpowder or metal fiber including copper, nickel, aluminum, silver, and/orthe like; a conductive polymer such as a polyphenylene derivative; andmixtures thereof.

The negative electrode 112 includes a negative collector and a negativeactive material layer disposed on the negative current collector.

The negative current collector may be a copper foil.

The negative active material layer may include a negative activematerial, a binder, and optionally a conductive material.

The negative active material may include a material that reversiblyintercalates/deintercalates lithium ions, lithium metal, a lithium metalalloy, a material being capable of doping/dedoping lithium, or atransition metal oxide.

The material that reversibly intercalates/deintercalates lithium ionsmay be a carbon material. The carbon material may be any carbon-basednegative active material that is generally used in a lithium ionrechargeable battery. Examples of the carbon material includecrystalline carbon, amorphous carbon, and mixtures thereof. Thecrystalline carbon may be non-shaped, or sheet, flake, spherical, orfiber shaped natural graphite or artificial graphite. The amorphouscarbon may be a soft carbon, a hard carbon, a mesophase pitch carbonizedproduct, fired coke, and/or the like.

Examples of the lithium metal alloy include lithium and a metal selectedfrom Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge,Al, and/or Sn.

Examples of the material being capable of doping/dedoping lithiuminclude a Si-based compound such as Si, SiO_(x) (0<x<2), a Si—Ccomposite, a Si-Q alloy (wherein Q is not Si and is an alkali metal, analkaline-earth metal, a Group 13 to 16 element, a transition element, arare earth element, or a combination thereof), a Si—C composite, or acombination thereof; a Sn-based compound such as Sn, SnO₂, a Sn—Ccomposite, a Sn—R alloy (wherein R is not Sn and is an alkali metal, analkaline-earth metal, a Group 13 to 16 element, a transition element, arare earth element, or a combination thereof), or a combination thereof;or a combination thereof. At least one of these materials may be mixedwith SiO₂. The elements Q and R may be selected from Mg, Ca, Sr, Ba, Ra,Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb,Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti,Ge, P, As, Sb, Bi, S, Se, Te, Po, or a combination thereof.

Examples of the transition metal oxide include vanadium oxide, lithiumvanadium oxide, and the like.

The binder improves binding properties of negative active materialparticles with one another and with a current collector. Examples of thebinder include polyvinylalcohol, carboxylmethylcellulose,hydroxypropylcellulose, polyvinylchloride, carboxylatedpolyvinylchloride, polyvinylfluoride, an ethylene oxide-containingpolymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene, astyrene-butadiene rubber, an acrylated styrene-butadiene rubber, anepoxy resin, nylon, and/or the like, but the binder is not limitedthereto.

The conductive material is included to improve electrode conductivity.Any electrically conductive material may be used as a conductivematerial unless it causes a chemical change. Examples of the conductivematerial include a carbon-based material such as natural graphite,artificial graphite, carbon black, acetylene black, ketjen black, acarbon fiber, and/or the like; a metal-based material of metal powder ormetal fiber including copper, nickel, aluminum, silver, and/or the like;a conductive polymer such as a polyphenylene derivative; or a mixturethereof.

Each of the negative and positive electrodes 112 and 114 may befabricated by mixing the active material, a conductive material, and abinder to prepare an active material composition and coating thecomposition on a current collector.

Electrode manufacturing methods are well known to those of ordinaryskill in the art, and thus, they are not described in detail in thepresent specification. The solvent may include N-methylpyrrolidone orthe like, however, it is not limited thereto.

The electrolyte includes a non-aqueous organic solvent and a lithiumsalt.

The non-aqueous organic solvent serves as a medium for transmitting ionstaking part in the electrochemical reaction of a battery. Thenon-aqueous organic solvent may be one or more of a carbonate-basedsolvent, an ester-based solvent, an ether-based solvent, a ketone-basedsolvent, an alcohol-based solvent, or an aprotic solvent.

The carbonate-based solvent may include, for example, dimethyl carbonate(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropylcarbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate(MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate (BC), and/or the like.

When the linear carbonate compounds and cyclic carbonate compounds aremixed, an organic solvent having high dielectric constant and lowviscosity can be provided. The cyclic carbonate and the linear carbonatemay be mixed together in a volume ratio ranging from about 1:1 to about1:9.

Examples of the ester-based solvent may include n-methylacetate,n-ethylacetate, n-propylacetate, dimethylacetate, methylpropionate,ethylpropionate, γ-butyrolactone, decanolide, valerolactone,mevalonolactone, caprolactone, and/or the like. Examples of theether-based solvent include dibutyl ether, tetraglyme, diglyme,dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and/or thelike. Examples of the ketone-based solvent include cyclohexanone and/orthe like. Examples of the alcohol-based solvent include ethyl alcohol,isopropyl alcohol, and/or the like.

The non-aqueous organic solvent may be used singularly or in a mixture.When the organic solvent is used in a mixture, the mixture ratio can becontrolled in accordance with a desired battery performance.

The non-aqueous electrolyte may further include an overcharge inhibitoradditive such as ethylenecarbonate, pyrocarbonate, and/or the like.

The lithium salt is dissolved in the organic solvent. The lithium saltsupplies lithium ions in the battery and improves lithium iontransportation between positive and negative electrodes therein.

The lithium salt may include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆,LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂), (where x and y are naturalnumbers), LiCl, Lil, LiB(C₂O₄)₂ (lithium bis(oxalato) borate, LiBOB), ora combination thereof, as a supporting electrolytic salt.

The lithium salt may be used in a concentration ranging from about 0.1 Mto about 2.0 M. In some embodiments, when the lithium salt is includedwithin the above concentration range, an electrolyte has goodperformance and good lithium ion mobility due to optimal electrolyteconductivity and viscosity.

The separator 113 may include any materials commonly used in theconventional lithium battery as long as it separates a negativeelectrode 112 from a positive electrode 114 and transports lithium ions.In other words, the separator 113 may have a low resistance to iontransportation and a high electrolyte impregnation. For example, theseparator may be selected from glass fiber, polyester, TEFLON(tetrafluoroethylene), polyethylene, polypropylene,polytetrafluoroethylene (PTFE), or a combination thereof. It may have aform of a non-woven fabric or a woven fabric. For example, apolyolefin-based polymer separator such as polyethylene, polypropylene,or the like is commonly used for a lithium ion battery. In order toprovide heat resistance and/or mechanical strength, a coated separatorincluding a ceramic component or a polymer material may be used. Theseparator may have a single-layered structure or multi-layeredstructure.

The following examples illustrate the present invention in more detail.These examples, however, should not in any sense be interpreted as beingintended to limit the scope of the present invention.

Those of ordinary skill in this art should be able to understandportions of the present disclosure that are not described in detail.

Preparation of Positive Active Material

Example 1

2.4 M aqueous solutions of NiSO₄, CoSO₄, and MnSO₄ were mixed in a moleratio of 4:3:3. A 7.5 M aqueous solution of NaOH and a 15 M aqueoussolution of NH₄OH were added thereto. The mixture was continuously mixedusing a co-precipitator. The resulting mixture was co-precipitated at40° C. at a reaction speed of about 700 rpm, and at a pH of 11 for 8hours, obtaining a (Ni_(0.4)Co_(0.3)Mn_(0.3))(OH)₂ precursor.

The precursor was washed with water, dried in a 120° C. oven, andfiltered. The obtained precursor was mixed with Li₂CO₃ in a weight ratioof about 1:1.3 using a mixer. The resulting mixture was raised to atemperature of 890° C. at a speed of 2° C./min and then fired at 890° C.for about 10 hours, preparing an over lithiated oxide,0.3Li₂MnO₃.0.7LiNi_(0.58)Co_(0.418)Mn_(0.002)O₂ (lithium metal oxide,Li_(1.3)Ni_(0.44)Co_(0.28)Mn_(0.28)O₂).

The over lithiated, 2.4 M aqueous solutions of NiSO₄, CoSO₄, and MnSO₄were mixed in a mole ratio of 6:2:2. A 7.5 M aqueous solution of NaOHand a 15 M aqueous solution of NH₄OH were added thereto. The mixture wascontinuously mixed using a co-precipitator. The resulting mixture wasco-precipitated at a pH of 11, 40° C., and a reaction speed of about 700rpm for 8 hours, obtaining a (Ni_(0.44)Co_(0.28)Mn_(0.28))(OH)₂precursor on a surface of the over lithiated oxide.

The resultant was washed with water, dried in a 120° C. oven, andfiltered. The obtained precursor was mixed with Li₂CO₃ in a weight ratioof about 1:1.08 using a mixer. The resulting mixture was raised to atemperature of 890° C. at a speed of 2° C./min and fired at 890° C. forabout 10 hours, obtaining a lithium metal oxide,Li_(1.3)Ni_(0.44)Co_(0.28)Mn_(0.28)O₂.

Comparative Example 1

2.4 M aqueous solutions of NiSO₄, CoSO₄, and MnSO₄ were mixed in a moleratio of 4:3:3. A 7.5 M aqueous solution of NaOH and a 15 M aqueoussolution of NH₄OH were added thereto. The mixture was continuously mixedusing a co-precipitator. The resulting mixture was co-precipitated at apH of 11, 40° C., and a reaction speed of about 700 rpm for 8 hours,obtaining a (Ni_(0.4)Co_(0.3)Mn_(0.3))(OH)₂ precursor.

The precursor was washed with water, dried in a 120° C. oven, andfiltered. The filtered precursor was mixed with Li₂CO₃ in a weight ratioof about 1:1.3 using a mixer. The mixture was raised to a temperature of890° C. at a speed of 2° C./min and then fired at 890° C. for about 10hours, obtaining over lithiated oxide,0.3Li₂MnO₃.0.7LiNi_(0.58)Co_(0.418)Mn_(0.002)O₂ (lithium metal oxide,Li_(1.3)Ni_(0.4)Co_(0.3)Mn_(0.3)O₂).

EVALUATION 1 Scanning Electronic Microscope (SEM) Photograph of PositiveActive Material

Scanning electron microscope (SEM) photographs were taken of thepositive active materials according to Example 1 and ComparativeExample 1. The results are provided in FIGS. 3 and 4.

FIGS. 3 and 4 are scanning electronic microscope (SEM) photographs ofthe positive active materials according to Example 1 and ComparativeExample 1. In FIG. 3, the white portions indicated the uncontinuous(non-continuous) portions of the lithium nickel cobalt manganese oxide.Referring to FIGS. 3 and 4, the positive active material according toExample 1 has a structure where the surface of the over lithiated oxideis coated in an island shape, i.e., it includes non-continuous portionson its surface, while the positive active material according toComparative Example 1 has a non-coated structure.

EVALUATION 2 Energy Dispersive X-Ray Spectrometer (EDX) Measurement ofPositive Active Material

An elemental analysis of the positive active material according toExample 1 was performed using an energy dispersive X-ray spectrometer(EDX). The result is provided in FIGS. 5 and 6.

FIG. 5 is an energy dispersive X-ray spectrometer (EDX) graph of theinside (i.e., the interior over-litihiated portion) of the positiveactive material according to Example 1, and FIG. 6 is an energydispersive X-ray spectrometer (EDX) graph showing the surface (i.e., theactive material on an outer surface of the over-lithiated portion) ofthe positive active material according to Example 1.

Based on the EDX graphs of FIGS. 5 and 6, the atomic amount on thesurface and the inside of the positive active material was analyzed, andthe result is provided in the following Table 1.

TABLE 1 Inside positive Surface of positive active material activematerial Ni (atom %) 50 63 Co (atom %) 20 20 Mn (atom %) 30 17

Referring to Table 1, the positive active material according to Example1 included Ni:Mn in an atomic weight ratio of between 1:1 to 2:1 insidethereof and in an atomic weight ratio of between 3:1 to 4:1 on thesurface. Accordingly, the positive active material was coated with alithium nickel cobalt manganese oxide that included a relatively largeamount of Ni and thus, had improved electrical conductivity on thesurface.

EVALUATION 3 Electrical Conductivity Measurement of Positive ActiveMaterial

The positive active materials according to Example 1 and ComparativeExample 1 were measured regarding electrical conductivity, and theresult is provided in the following Table 2.

The electrical conductivity was measured using a 4 probe chip byapplying a pressure of 20 kN to a positive active material and formingit into a pellet.

TABLE 2 Example 1 Comparative Example 1 Electrical conductivity (S/cm)5.3 × 10⁻² 7.06 × 10⁻³

Referring to Table 2, the positive active material according to Example1 including non-continuous portions of lithium nickel cobalt manganeseoxide (i.e., island shapes) coated on the surface of the over lithiatedoxide had higher electrical conductivity than the positive activematerial having a non-coated structure according to Comparative Example1.

EVALUATION 4 XRD Measurement of Positive Active Material

The positive active material according to Example 1 was measured usingan X-ray diffraction analysis (XRD). The result is provided in thefollowing FIG. 7.

FIG. 7 is an X-ray diffraction analysis (XRD) graph showing the positiveactive material according to Example 1. Referring to FIG. 7, the graphhad a main Li₂MnO₃ peak around about 22° C.

Fabrication of Rechargeable Lithium Battery Cells

96 wt % of each positive active material according to Example 1 andComparative Example 1, 2 wt % of polyvinylidene fluoride (PVdF), and 2wt % of acetylene black were mixed, and the mixtures were dispersed inN-methyl-2-pyrrolidone, preparing slurries. The slurries wererespectively coated on 60 μm-thick aluminum foil, dried at 135° C. forat least about 3 hours, and compressed, fabricating positive electrodeswith a positive active material layer.

The positive electrode and a lithium metal counter electrode were usedto fabricate a coin-type half-cell. Herein, an electrolyte solution wasprepared by mixing ethylenecarbonate (EC) and dimethylcarbonate (DMC) ina volume ratio of 3:7 and dissolving LiPF₆ therein at a concentration of1.3 M.

EVALUATION 5 Charge and Discharge Characteristic of Rechargeable LithiumBattery Cell

Each half-cell fabricated using each positive active material accordingto Example 1 and Comparative Example 1 was evaluated regarding chargeand discharge characteristics. The results are provided in FIGS. 8 and9.

The half-cells were charged and discharged at a charge rate of 0.1 Cduring the first cycle. Herein, the charge had a cut-off voltage of 4.7V, and the discharge had a cut-off voltage of 2.0 V. Starting with thesecond cycle, the half-cells were charged and discharged with a chargerate of 0.2 C, 0.5 C, 1 C, and 2 C. Herein, the charge had a cut-offvoltage of 4.6 V, and the discharge had a cut-off voltage of 2.0 V.

FIGS. 8 and 9 are charge and discharge graphs of the half-cellsincluding each positive active material according to Example 1 andComparative Example 1.

Referring to FIGS. 8 and 9, the half-cell using the positive activematerial including non-continuous portions of lithium nickel cobaltmanganese oxide on the surface of the over lithiated oxide according toExample 1 had good high rate charge and discharge characteristicscompared with the cell using the positive active material according toComparative Example 1.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, and equivalents thereof.

What is claimed is:
 1. A positive active material for a rechargeablelithium battery, comprising a nickel-based composite oxide representedby the following Chemical Formula 1, wherein the nickel-based compositeoxide comprises an over lithiated oxide and non-continuous portions of alithium nickel cobalt manganese oxide on a surface of the over lithiatedoxide, wherein an atomic weight ratio of Ni:Mn of the over lithiatedoxide is in a range of about 1:1 to about 2:1, wherein an atomic weightratio of Ni:Mn of the lithium nickel cobalt manganese oxide is in arange of about 3:1 to about 4:1:Li_(a)Ni_(b)Co_(c)Mn_(d)O₂  Chemical Formula 1 wherein, 1<a<1.6,0.1<b<0.7, 0.1<c<0.4, and 0.1<d<0.7, and wherein the manganese andnickel components of the nickel-based composite oxide have a structuralconcentration gradient such that the nickel-based composite oxide has ahigher concentration of nickel at a surface than at an interior.
 2. Thepositive active material for a rechargeable lithium battery of claim 1,wherein the over lithiated oxide comprises a compound represented by thefollowing Chemical Formula 2:xLi₂MnO₃.(1-x)LiNi_(e)Co_(f)Mn_(g)O₂  Chemical Formula 2 wherein, 0<x<1,0<e<1, 0<f<1, 0<g<1, and e+f+g=1.
 3. The positive active material for arechargeable lithium battery of claim 1, wherein the over lithiatedoxide is included in an amount of about 5 wt % to about 60 wt % based onthe total weight of the nickel-based composite oxide.
 4. The positiveactive material for a rechargeable lithium battery of claim 1, whereinthe over lithiated oxide is included in an amount of about 15 wt % toabout 50 wt % based on the total weight of the nickel-based compositeoxide.
 5. The positive active material for a rechargeable lithiumbattery of claim 1, wherein the lithium nickel cobalt manganese oxidecomprises a compound represented by the following Chemical Formula 3:Li_(i)Ni_(j)Co_(k)Mn_(l)O₂  Chemical Formula 3 wherein, 0.95<i<1.05,0.4≦j≦0.8, 0.1≦k≦0.3, and 0.1≦l≦0.3.
 6. The positive active material fora rechargeable lithium battery of claim 1, wherein the lithium nickelcobalt manganese oxide is included in an amount of about 40 wt % toabout 95 wt % based on the total weight of the nickel-based compositeoxide.
 7. The positive active material for a rechargeable lithiumbattery of claim 1, wherein the lithium nickel cobalt manganese oxide isincluded in an amount of about 50 wt % to about 85 wt % based on thetotal weight of the nickel-based composite oxide.
 8. The positive activematerial for a rechargeable lithium battery of claim 1, wherein anatomic weight ratio of Ni:Mn of the over lithiated oxide is in a rangeof about 1:1 to about 1.7:1.
 9. The positive active material for arechargeable lithium battery of claim 1, wherein an atomic weight ratioof Ni:Mn of the lithium nickel cobalt manganese oxide is in a range ofabout 3:1 to about 3.5:1.
 10. A rechargeable lithium battery, comprisinga positive electrode comprising the positive active material accordingto claim 1; a negative electrode; and an electrolyte.